Rfid tag having capacitive load

ABSTRACT

The present invention relates to an RFID tag having capacitive load, including: an antenna unit, a capacitive load and an RFID device. The antenna unit includes two metal sheets and a conductive substrate, and the metal sheets are disposed at a corresponding position above the conductive substrate and electrically connected to the conductive substrate. The capacitive load is electrically connected to the metal sheets. The RFID device feeds the antenna unit by electrically connecting to the metal sheets or coupling. This dramatically decreases the operating frequency, so the imaginary part of impedance is higher with the same size of the antenna unit, thereby reducing the size of the RFID tag. As a result, the weight is reduced, the manufacture process is simplified and the subsequent structure can be easily adjusted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an RFID tag, and more particularly toan RFID tag having capacitive load.

2. Description of the Related Art

RFID (Radio frequency Identification) tags are directly attached toproducts, so it will encounter many problems during application. Inconventional technology, the system cannot read the identificationinformation in RFID tags in the case of, for example, products packagedwith metal material, products with liquid content and so on. Especiallyin application with metal items, as metal is an ideal conductor and aperfect reflecting surface for electromagnetic waves, interference withsignals often occurs when RFID tags are attached to metal items, andantennas close to the metal items cannot effectively receive andtransmit electromagnetic waves; as a result, the effective readingdistance of the system is decreased from meters to 1-2 centimeters, orthe system totally fails.

In order to apply RFID technology to metal items such as automotivecomponents, containers, industrial equipment and so on, specialized RFIDtags have been developed, but such RFID tags in conventional technologyare too big in size and costly. Therefore, developing specialized RFIDtags for use with metal which are small and cost-efficient is a goalthat has not yet been achieved.

Moreover, recent development in RFID technology is toward item-level, sominiaturization is desirable in the design of RFID tags. In conventionaldesign of RFID tags at item-level, providing the necessary inductivereactance is the main purpose, and this conductive design totallydepends on the length of a current loop; because of the limitation byphysical characteristics, highly inductive tag antennas are relativelylarge in size, which goes against the requirement for furtherminiaturization.

Therefore, it is necessary to provide an RFID tag having capacitive loadto solve the above problems.

SUMMARY OF THE INVENTION

The present invention is directed to an RFID (Radio frequencyIdentification) tag having capacitive load, comprising: an antenna unit,a capacitive load and an RFID device. The antenna unit includes twometal sheets and a conductive substrate, and the metal sheets aredisposed at a corresponding position above the conductive substrate andelectrically connected to the conductive substrate. The capacitive loadis electrically connected to the metal sheets. The RFID device feeds theantenna unit by electrically connecting to the metal sheets or coupling.

The RFID tag having capacitive load according to the present inventioncan dramatically decrease the operating frequency, so the imaginary partof impedance is higher with the same size of the antenna unit, therebyreducing the size of the RFID tag. As a result, the weight is reduced,the manufacture process is simplified and the subsequent structure canbe easily adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an RFID tag having capacitive loadaccording to a first embodiment of the present invention;

FIGS. 2 to 7 are schematic views of five different forms of the RFID taghaving capacitive load according to the first embodiment of the presentinvention;

FIG. 8 is a schematic view of an RFID tag having capacitive loadaccording to a second embodiment of the present invention;

FIG. 9 is a schematic view of characteristic impedance and operatingfrequency of an RFID tag; and

FIG. 10 is a schematic view of a comparison between the characteristicimpedance of an RFID tag having capacitive load according to the presentinvention and that of an RFID tag without capacitive load.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1( a) to 1(c) and FIGS. 1( a′) to 1(c′) show schematic views oftwo forms of an RFID (Radio frequency Identification) tag havingcapacitive load according to a first embodiment of the presentinvention. FIG. 1( a) and FIG. 1( a′) are top views; FIG. 1( b) and FIG.1( b′) are front views; and FIG. 1( c) and FIG. 1( c′) are right views.Thus, in the following figures, FIG. (a) and FIG. (a′) indicate topviews, FIG. (b) and FIG. (b′) indicate front views, and FIG. (c) andFIG. (c′) indicate right views.

In this embodiment shown in FIGS. 1( a) to 1(c), the RFID tag 1 havingcapacitive load comprises: an antenna unit 11, a capacitive load 12 andan RFID device 13. The antenna unit 11 includes two metal sheets 111 anda conductive substrate 112, and the metal sheets 111 are disposed at acorresponding position above the conductive substrate 112 andelectrically connected to the conductive substrate 112.

In this embodiment, the antenna unit 11 further includes two conductiveportions 113, and the conductive portions 113 are electrically connectedto the metal sheets 111 and the conductive substrate 112 respectively.The conductive portions 113 may be conductive pillars or conductivesheets. Each metal sheet 111 includes a first side 114 and a second side115 corresponding to the first side 114, and the first sides 114 of themetal sheets 111 are close to and face each other, so that the metalsheets 111 are spaced apart from each other at a distance.

In this embodiment, the conductive portions 113 are conductive pillars,the conductive pillars approach the second sides 115 of the metal sheets111 and are disposed between the metal sheets 111 and the conductivesubstrate 112 (see FIGS. 1( b) to 1(c)). In other applications, theconductive portions 113 may be conductive sheets, and the conductivesheets are electrically connected to the second sides 115 of the metalsheets 111 and two corresponding sides of the conductive substrate 112(see FIGS. 1( b′) to 1(c′)).

In this embodiment, the capacitive load 12 crosses the first sides 114and is electrically connected to the metal sheets 111, and the RFIDdevice 13 crosses the first sides 114 and is electrically connected tothe metal sheets 111. It should be noted that, in other applications,the RFID device 13 may include an RFID chip 131 and a metal strap 132,wherein two ends of the metal strap 132 are connected to two ends of theRFID chip 131 to form a loop structure, and the RFID device 13 isdisposed in a notch 110 formed between the metal sheets 111 and feedsthe antenna unit 11 by coupling (as shown in FIG. 2).

Alternatively, the RFID device 13 may further include an RFID chip 133and two flank metal sheets 134, wherein the flank metal sheets 134 aredisposed on a surface of a dielectric layer 14, the RFID chip 133 iselectrically connected to the flank metal sheets 134, the RFID device 13is spaced apart from the metal sheets 111 by the dielectric layer 14 anddisposed at a corresponding position above the metal sheets 111, and theRFID device 13 feeds the antenna unit 11 by coupling (as shown in FIG.3).

In FIGS. 1( a) to 1(c), the capacitive load 12 is an SMD (Surface MountDevice) capacitor element. It should be noted that the capacitive load12 may include a load dielectric material 121 and a metal layer 122, andthe load dielectric material 121 is disposed between the metal sheets111 and the metal layer 112 (as shown in FIG. 4).

Alternatively, the capacitive load 12 may further comprise four firstextension sheets 123, wherein each first extension sheet 123 isconnected to a third side 116 of the metal sheet 111 at place near thefirst side 114 and extends along the direction approaching the secondside 115, and each first extension sheet 123 is essentially parallel tothe third side 116. Two corresponding third sides 116 of each metalsheet 111 are located between the first side 114 and the second side 115(as shown in FIG. 5). Each first extension sheet 123 and part of themetal sheet 111 disposed on the third side 116 produce capacitive load.

Alternatively, the capacitive load 12 may further comprise two secondextension sheets 124, wherein the second extension sheets 124 aredisposed on the same side of the metal sheets 111, each second extensionsheet 124 is connected to a third side 116 of the metal sheet 111 atplace near the first side 114, and each second extension sheet 124extends along the direction approaching the second side 115 which isessentially parallel to the third side 116, backward along the directionapproaching the first side 114, and further toward the second side 115on the other side. The second extension sheets 124 extend crisscross toform a finger-crossed structure (as shown in FIG. 6).

Alternatively, the capacitive load 12 may comprise two second extensionsheets 124 and two third extension sheets 125, wherein the thirdextension sheets 125 and the second extension sheets 124 are disposed ontwo corresponding sides of the metal sheets 111, each third extensionsheet 125 is connected to a third side 116 of the metal sheet 111 atplace near the first side 114, and each third extension sheet 125extends along the direction approaching the second side 115 which isessentially parallel to the third side 116, backward along the directionapproaching the first side 114, and further toward the second side 115on the other side. The second extension sheets 124 and the thirdextension sheets 125 extend crisscross to form a finger-crossedstructure (as shown in FIG. 7).

FIGS. 8( a) to 8(c) and FIGS. 8( a′) to 8(c′) show schematic views oftwo forms of an RFID tag having capacitive load according to a secondembodiment of the present invention. The RFID tag 2 having capacitiveload according to the embodiment is substantially the same as the RFIDtag 1 having capacitive load according to the first embodiment (FIG. 7),with the only difference in the structure of the antenna unit 21. Inthis embodiment, the antenna unit 21 further includes at least onesubstrate metal layer 211 and a substrate dielectric material 212, theat least one substrate metal layer 211 is disposed between the metalsheets 111 and the conductive substrate 112, and the substratedielectric material 212 is disposed between the at least one substratemetal layer 211 and the metal sheets 111 and between the at least onesubstrate metal layer 211 and the conductive substrate 112. The otherparts of the RFID tag 2 having capacitive load which are the same as theRFID tag 1 having capacitive load according to the first embodiment(FIG. 7) are designated by the same reference numbers, and therefore thedescription is skipped.

It is noted that the conductive portions 213 of the antenna unit 21 maybe conductive pillars or conductive sheets. The antenna unit 21 may alsofurther include the structure of any kind of capacitive loads 12 asshown in FIG. 4, 5 or 6.

The RFID tags 1, 2 having capacitive load according to the presentinvention may be designed to have the frequency band of UHF(Ultrahigh-frequency), 2.45 GHz or 5.8 GHz, and may be disposed on thesurface of metal material products for management (e.g. inventorymanagement) of metal material products.

FIG. 9( a) shows a schematic view of characteristic impedance of an RFIDtag; FIG. 9( b) shows a schematic view of operating frequency of an RFIDtag. In FIG. 9( a), the operating frequency of the RFID tag falls onF_(cAnt) (about 1500 MHz); in FIG. 9( b), the goal operating frequencyof the RFID tag falls on F_(cTag) (about 930 MHz). The RFID tags havingcapacitive load according to the present invention have capacitive load,which dramatically decreases the operating frequency thereof and make itfall on the goal operating frequency F_(cTag).

FIG. 10 shows a schematic view of characteristic impedance comparing anRFID tag having capacitive load according to the present invention withan RFID tag without capacitive load. The dotted line L1 is the curve ofthe real part of impedance of the RFID tag having capacitive loadaccording to the present invention, and the solid line L2 is the curveof the real part of impedance of the RFID tag having capacitive loadaccording to the present invention; the dotted line L3 is the curve ofthe real part of impedance of the RFID tag without capacitive load, andthe solid line L4 is the curve of the real part of impedance of the RFIDtag without capacitive load.

As shown in FIG. 10, the imaginary part of impedance of the RFID tagwithout capacitive load is about 1000Ω, and the operating frequencyF_(cAnt) thereof falls on about 1900 MHz; as shown in FIG. 10, theimaginary part of impedance of the RFID tag having capacitive loadaccording to the present invention is increased to about 1200Ω, and theoperating frequency F_(cAnt) _(—) _(c) thereof is dramatically decreasedto about 980 MHz. Moreover, as a comparison of the dotted line L1 withthe dotted line L3 shows, the real part of impedance of the RFID taghaving capacitive load according to the present invention is alsogreater than that of the RFID tag without capacitive load.

As described above, the RFID tags having capacitive load according tothe present invention can dramatically decrease the operating frequency,so the imaginary part of impedance is higher with the same size of theantenna unit, thereby reducing the size of the RFID tag. As a result,the weight is reduced, the manufacture process is simplified (massproduction directly by PCB process is possible) and the subsequentstructure can be easily adjusted.

While several embodiments of the present invention have been illustratedand described, various modifications and improvements can be made bythose skilled in the art. The embodiments of the present invention aretherefore described in an illustrative and not restrictive sense.

It is intended that the present invention should not be limited to theparticular forms as illustrated, and that all modifications whichmaintain the spirit and scope of the present invention are within thescope defined in the appended claims.

1. An RFID tag having capacitive load, comprising: an antenna unit,including two metal sheets and a conductive substrate, wherein the metalsheets are disposed at a corresponding position above the conductivesubstrate and electrically connected to the conductive substrate; acapacitive load, electrically connected to the metal sheets; and an RFIDdevice, feeding the antenna unit by electrically connecting to the metalsheets or coupling.
 2. The RFID tag as claimed in claim 1, wherein theantenna unit further includes two conductive portions electricallyconnected to the metal sheets and the conductive substrate respectively.3. The RFID tag as claimed in claim 2, wherein the conductive portionsare conductive pillars or conductive sheets.
 4. The RFID tag as claimedin claim 3, wherein each metal sheet includes a first side and a secondside corresponding to the first side, the first sides of the metalsheets are close to and face each other, and the conductive pillarsapproach the second sides of the metal sheets respectively and aredisposed between the metal sheets and the conductive substrate.
 5. TheRFID tag as claimed in claim 3, wherein each metal sheet includes afirst side and a second side corresponding to the first side, the firstsides of the metal sheets are close to and face each other, and theconductive sheets are electrically connected to the second sides of themetal sheets and two corresponding sides of the conductive substrate. 6.The RFID tag as claimed in claim 1, wherein the antenna unit furtherincludes at least one substrate metal layer and a substrate dielectricmaterial, the at least one substrate metal layer is disposed between themetal sheets and the conductive substrate, and the substrate dielectricmaterial is disposed between the at least one substrate metal layer andthe metal sheets and between the at least one substrate metal layer andthe conductive substrate.
 7. The RFID tag as claimed in claim 1, whereina notch is formed between the metal sheets, the RFID device furtherincludes an RFID chip and a metal strap, two ends of the metal strap areconnected to two ends of the RFID chip to form a loop structure, and theRFID device is disposed in the notch formed between the metal sheets andfeeds the antenna unit by coupling.
 8. The RFID tag as claimed in claim1, further comprising a dielectric layer, wherein the RFID devicefurther includes an RFID chip and two flank metal sheets, the flankmetal sheets are disposed on a surface of the dielectric layer, the RFIDchip is electrically connected to the flank metal sheets, the RFIDdevice is spaced apart from the metal sheets by the dielectric layer anddisposed at a corresponding position above the metal sheets, and theRFID device feeds the antenna unit by coupling.
 9. The RFID tag asclaimed in claim 1, wherein the capacitive load is an SMD (Surface MountDevice) capacitor element.
 10. The RFID tag as claimed in claim 3,wherein the capacitive load is an SMD capacitor element.
 11. The RFIDtag as claimed in claim 6, wherein the capacitive load is an SMDcapacitor element.
 12. The RFID tag as claimed in claim 7, wherein thecapacitive load is an SMD capacitor element.
 13. The RFID tag as claimedin claim 8, wherein the capacitive load is an SMD capacitor element. 14.The RFID tag as claimed in claim 1, wherein the capacitive load includesa load dielectric material and a load metal layer, and the loaddielectric material is disposed between the metal sheets and the loadmetal layer.
 15. The RFID tag as claimed in claim 3, wherein thecapacitive load includes a load dielectric material and a load metallayer, and the load dielectric material is disposed between the metalsheets and the load metal layer.
 16. The RFID tag as claimed in claim 6,wherein the capacitive load includes a load dielectric material and aload metal layer, and the load dielectric material is disposed betweenthe metal sheets and the load metal layer.
 17. The RFID tag as claimedin claim 7, wherein the capacitive load includes a load dielectricmaterial and a load metal layer, and the load dielectric material isdisposed between the metal sheets and the load metal layer.
 18. The RFIDtag as claimed in claim 8, wherein the capacitive load includes a loaddielectric material and a load metal layer, and the load dielectricmaterial is disposed between the metal sheets and the load metal layer.19. The RFID tag as claimed in claim 1, wherein the capacitive loadfurther comprises four first extension sheets, each metal sheet includesa first side, a second side corresponding to the first side and twocorresponding third sides, the first sides of the metal sheets are closeto and face each other, two third sides of each metal sheet are locatedbetween the first side and the second side, each first extension sheetis connected to the third side at place near the first side and extendsalong the direction approaching the second side, and each firstextension sheet is essentially parallel to the third side.
 20. The RFIDtag as claimed in claim 3, wherein the capacitive load further comprisesfour first extension sheets, each metal sheet includes a first side, asecond side corresponding to the first side and two corresponding thirdsides, the first sides of the metal sheets are close to and face eachother, two third sides of each metal sheet are located between the firstside and the second side, each first extension sheet is connected to thethird side at place near the first side and extends along the directionapproaching the second side, and each first extension sheet isessentially parallel to the third side.
 21. The RFID tag as claimed inclaim 6, wherein the capacitive load further comprises four firstextension sheets, each metal sheet includes a first side, a second sidecorresponding to the first side and two corresponding third sides, thefirst sides of the metal sheets are close to and face each other, twothird sides of each metal sheet are located between the first side andthe second side, each first extension sheet is connected to the thirdside at place near the first side and extends along the directionapproaching the second side, and each first extension sheet isessentially parallel to the third side.
 22. The RFID tag as claimed inclaim 7, wherein the capacitive load further comprises four firstextension sheets, each metal sheet includes a first side, a second sidecorresponding to the first side and two corresponding third sides, thefirst sides of the metal sheets are close to and face each other, twothird sides of each metal sheet are located between the first side andthe second side, each first extension sheet is connected to the thirdside at place near the first side and extends along the directionapproaching the second side, and each first extension sheet isessentially parallel to the third side.
 23. The RFID tag as claimed inclaim 8, wherein the capacitive load further comprises four firstextension sheets, each metal sheet includes a first side, a second sidecorresponding to the first side and two corresponding third sides, thefirst sides of the metal sheets are close to and face each other, twothird sides of each metal sheet are located between the first side andthe second side, each first extension sheet is connected to the thirdside at place near the first side and extends along the directionapproaching the second side, and each first extension sheet isessentially parallel to the third side.
 24. The RFID tag as claimed inclaim 1, wherein the capacitive load further comprises two secondextension sheets, each metal sheet includes a first side, a second sidecorresponding to the first side and two corresponding third sides, thefirst sides of the metal sheets are close to and face each other, twothird sides of each metal sheet are located between the first side andthe second side, the second extension sheets are disposed on the sameside of the metal sheets, each second extension sheet is connected tothe third side at place near the first side, each second extension sheetextends along the direction approaching the second side which isessentially parallel to the third side and extends backward along thedirection approaching the first side, and the second extension sheetsextend crisscross to form a finger-crossed structure.
 25. The RFID tagas claimed in claim 3, wherein the capacitive load further comprises twosecond extension sheets, each metal sheet includes a first side, asecond side corresponding to the first side and two corresponding thirdsides, the first sides of the metal sheets are close to and face eachother, two third sides of each metal sheet are located between the firstside and the second side, the second extension sheets are disposed onthe same side of the metal sheets, each second extension sheet isconnected to the third side at place near the first side, each secondextension sheet extends along the direction approaching the second sidewhich is essentially parallel to the third side and extends backwardalong the direction approaching the first side, and the second extensionsheets extend crisscross to form a finger-crossed structure.
 26. TheRFID tag as claimed in claim 6, wherein the capacitive load furthercomprises two second extension sheets, each metal sheet includes a firstside, a second side corresponding to the first side and twocorresponding third sides, the first sides of the metal sheets are closeto and face each other, two third sides of each metal sheet are locatedbetween the first side and the second side, the second extension sheetsare disposed on the same side of the metal sheets, each second extensionsheet is connected to the third side at place near the first side, eachsecond extension sheet extends along the direction approaching thesecond side which is essentially parallel to the third side and extendsbackward along the direction approaching the first side, and the secondextension sheets extend crisscross to form a finger-crossed structure.27. The RFID tag as claimed in claim 7, wherein the capacitive loadfurther comprises two second extension sheets, each metal sheet includesa first side, a second side corresponding to the first side and twocorresponding third sides, the first sides of the metal sheets are closeto and face each other, two third sides of each metal sheet are locatedbetween the first side and the second side, the second extension sheetsare disposed on the same side of the metal sheets, each second extensionsheet is connected to the third side at place near the first side, eachsecond extension sheet extends along the direction approaching thesecond side which is essentially parallel to the third side and extendsbackward along the direction approaching the first side, and the secondextension sheets extend crisscross to form a finger-crossed structure.28. The RFID tag as claimed in claim 8, wherein the capacitive loadfurther comprises two second extension sheets, each metal sheet includesa first side, a second side corresponding to the first side and twocorresponding third sides, the first sides of the metal sheets are closeto and face each other, two third sides of each metal sheet are locatedbetween the first side and the second side, the second extension sheetsare disposed on the same side of the metal sheets, each second extensionsheet is connected to the third side at place near the first side, eachsecond extension sheet extends along the direction approaching thesecond side which is essentially parallel to the third side and extendsbackward along the direction approaching the first side, and the secondextension sheets extend crisscross to form a finger-crossed structure.29. The RFID tag as claimed in claim 24, wherein the capacitive loadfurther comprises two third extension sheets, the third extension sheetsand the second extension sheets are disposed on two corresponding sidesof the metal sheets, each third extension sheet is connected to thethird side at place near the first side, each third extension sheetextends along the direction approaching the second side which isessentially parallel to the third side and extends backward along thedirection approaching the first side, and the third extension sheetsextend crisscross to form a finger-crossed structure.
 30. The RFID tagas claimed in claim 1, which is applied to inventory management of metalmaterial products.
 31. The RFID tag as claimed in claim 1, wherein thefrequency band comprises UHF (Ultrahigh-frequency), 2.45 GHz or 5.8 GHz.